Filament wire for use in the cathode of a thermionic valve



United States Patent 3,418,164 FILAMENT WIRE FOR USE IN THE CATHODE OF ATHERMIONIC VALVE Henri Provisor, Suresnes, France, assignor to NorthAmerican Philips Company, Inc., New York, N.Y., a corporation ofDelaware No Drawing. Continuation of application Ser. No.

331,762, Dec. 19, 1963. This application Jan. 19, 1968, Ser. No. 699,272Claims priority, application France, Feb. 6, 1963, 923,898 5 Claims.(Cl. 117215) ABSTRACT OF THE DISCLOSURE An indirectly heated cathodehaving a filament coated with alumina and a layer of zirconium, niobium,tantalum or titanium to reduce leakage and failure between the cathodeand filament.

This is a continuation of application Ser. No. 331,762, filed Dec, 19,1963, now abandoned.

The invention relates to a filament wire of an indirectly heatedcathode, the electron-emissive surface of which is formed by an alkalineearth oxide layer or a layer containing such oxides, said cathode beingemployed, as is known, in numerous types of thermionic tubes.

In accordance with the invention the risk of breakdown between thefilament and the cathode during the operation of the tubes is reduced,while the stray current between the filament and the cathode is lowerthan in the conventional tubes. The invention relates principally to theapplication of an additional layer of at least one element having anoxygen-binding power, which element does not reduce alumina at theoperational temperature of the filament and has a low vapor pressure,While the oxides of said element are resistant to alumina at the saidoperational temperatures, suitable elements to this end are Zr, Nb, Ta,and to -a lesser extent also Ti. The additional layer may be applieddirectly to the filament, i.e. beneath the alumina layer or to thisinsulating alumina layer.

The invention provides furthermore a method of applying a layer of thesaid elements and relates to thermionic tubes comprising an indirectlyheated cathode having a filament provided with such a layer.

Previous investigations have disclosed the role of oxygen in thedeterioration of the insulation of the alumina layer. It the voltageapplied between the filament and the cathode is such that the cathode isnegative with respect to the filament, the oxygen reacts with thetungsten, so that a volatile oxide is produced, which condenses anddecomposes at the surface of the alumina particles. The released oxygenagain attacks the tungsten wire, forms a new oxide molecule of thismetal, which volatilizes, condenses and again decomposes. The cycle isrepeated as long as the released oxygen is not bound somewhere in theform of a stable compound. The accumulation of metallic tungsten atomsfrom the oxide molecules between the alumina particles changes theinsulating resistance of the alumina. The production of metallicdeposits at the contact area between the filament and the cathode provesthat the tungsten is transferred in the form of positive ions. However,it is possible that the oxygen released by properties of the alumina.Among the elements providing a resistant oxide only those are suitablewhich have a low vapor pressure and the oxide of which does not formwith the alumina a compound of moderate electrical resistance or alow-melting point eutectic. Moreover, at the operational temperature theelement should not reduce the alumina. These requirements restrict thechoice to a few elements, for example zirconium, niobium, tantalum andtitanium.

The invention uses one of these elements or a few of them, independentlyof the theory set out above.

the decomposition of tungsten oxide should flow towards The use of thesaid elements in the form of the hydride is particularly simple, whichprovides a double advan tage: on the one hand the metal filaments can becoated by electrophoresis and on the other hand there is obtained aprotection of the elements strongly reacting with oxygen from oxidationduring the sintering of the alumina on the tungsten filament.

Zirconium hydride is deposited on a tungsten wire by cataphoresis from asuspension of gms. of ZrH in 2000 mls. of isopropyl alcohol and 40 mls.of NiCl -0.2 N. After drying of the deposit, the filament is coated withalumina in a conventional manner. The sintering of the alumina does notexhibit any visible change with the use of zirconium hydride whichsticks to the tungsten wire.

A filament provided with a layer of oxygen-binding elements on the outersurface of the insulating alumina sheath may be manufactured as follows.In a conventional manner the alumina layer is deposited on the tungsternwire. On this first coating there is deposited a alyer of the selectedhydride. At this same time the two layers are sintered at a hightemperature in a reducing atmosphere.

Satisfactory results are obtained by using titanium hydride. After theapplication of the alumina layer on the tungsten wire by cataphoresis, asuspension of a mixture of titanium and alumina in collodium was appliedby spraying. It has been found to be advantageous to use a suspension ofthe following composition: 100 gms. of A1 0 20 gms. of TiH in 200 mls.of collodium, diluted with mls. of methanol. The collodium was asolution of 2.50 gms. of nitro-cellulose in 75 gms of glycol ethyleneand 22.5 gms. of butanol.

The first alumina layer and the deposit obtained by spraying aresimultaneously sintered under the conditions usually prevailing insintering the insulating alumina sheath. This provides a black filamentwithout deterioration of the insulating properties of the alumina at thecontact area with the tungsten wire.

A filament thus manufactured combines the advantages of the presence ofan oxygen-binding element and of the thermal radiation of a black body.The heat transfer by radiation is materially greater owing to themetallic titanium released from the hydride.

I claim:

1. In an indirectly heated cathode for an electron discharge tube, saidcathode having an electron emissive surface constituted by a layer ofalkaline earth oxide, a filament wire for heating said cathode to atemperature at which the cathode emits electrons having thereon aninsulating layer of alumina between the wire and the electron emissivelayer and a layer adjacent to said insulating layer of an element havinga high-oxygen binding capacity and which does not reduce alumina at thetemperature at which the cathode emits electrons and which has a lowvapor pressure and an oxide which is resistant to alumina at saidtemperature, said element being selected from the group consisting ofzirconium, niobium, tantalum, and titanium.

2. A cathode as claimed in claim 1 in which the layer of the element isover the alumina layer.

3. A cathode as claimed in claim 1 in which the layer of the element isbetween the filament and the alumina layer.

4. A cathode as claimed in claim 2 in which the filament is tungsten.

5. A cathode as claimed in claim 4 in which the layer of the elementconsists of a hydride of the element.

References Cited 4 3,134,691 5/1964 Kopecky 117-215 3,161,540 12/1964Kingsley 61; a1. 117-217 3,195,004 7/1965 HaSSctt 117 231 X FOREIGNPATENTS 891,705 3/1962 Great Britain.

ALFRED L. LEAVITT, Primary Examiner.

C. K. WEIFFENBACH, Assistant Examiner.

US. Cl. X.R.

